High energy semiconductor radiation detectors provide detection of gamma rays and x-rays in numerous fields including nuclear instrumentation, medical imaging, biological research, astronomy, and dosimetry. However, known high energy scintillator-based detectors fail to provide acceptable energy resolution while operating at room temperature. For instance, the known high energy detectors are ill-suited for use in gamma ray spectrometers operating at room temperature.
Currently available Metal-Semiconductor-Metal (hereinafter "MSM") devices have proven useful for several spectroscopic applications. MSM detectors tend to have reasonably good energy resolution, typically 5% at 120 keV at room temperature. However, MSM detectors suffer from particular drawbacks. For instance, MSM detectors lack the energy resolution necessary to meet the requirements for spectroscopic applications. Additionally, the known MSM detectors are severely limited by their high inter pixel leakage current.
Others have attempted to overcome the drawbacks associated with MSM detectors by either increasing the bias voltage applied across the detectors or by cooling the detectors to a temperature less than -30.degree. C. Increasing the bias voltage across the MSM detectors, however, increases the leakage current through the detectors. The increased leakage current degrades the energy resolution of the detector. Additionally, cooling the MSM detectors, especially large arrays of detectors, proves to be problematic because of the quantities of power consumed to cool the detectors. An alternative to the MSM detector is the P-I-N detector.
The P-I-N design can offer low leakage current at large bias voltages, relative to MSM detectors. A basic P-I-N semiconductor radiation detector includes a wafer of intrinsic material with doped contacts formed on the opposite surfaces of the intrinsic layer. A reverse biasing electric field is applied across the contacts. High energy radiation, such as gamma rays, passing through the wafer of intrinsic material liberate electron-hole pairs which are swept to the respective contacts by the electric field and generate electrical pulses in an associated electronic unit.
Rhiger, U.S. Pat. No. 5,391,882, discloses a P-I-N type gamma ray detector having an intrinsic layer formed of cadmium telluride (CdTe) or cadmium zinc telluride (CdZnTe). The P-type and N-type semiconductor layers are formed of mercury cadmium telluride (HgCdTe). The intrinsic layer is a wide bandgap semiconductor detector layer, and the doped layers are graded such that the bandgap of the doped layers decreases with distance from the intrinsic layer.
The P-I-N detector disclosed in the Rhiger Patent is manufactured by growing a CdZnTe substrate and then forming HgCdTe layers by liquid phase epitaxially growth techniques. Even though this approach produces gamma ray detectors having higher energy resolution than MSM detectors, the fabrication method is expensive and is not useful for High Pressure Bridgman (HPB) CdZnTe because of the high temperatures used during the liquid phase epitaxially growth phase. Several researchers have observed that high temperatures (greater than 150 degrees C or perhaps lower, depending on the anneal time) severely degrade the resistivity and detector properties of HPB CdZnTe. Since liquid phase epitaxy growth of HgCdTe layers is typically done around 400 to 550 degrees C, this process is not suitable for HPB CdZnTe.
Another method known in the art for forming P-I-N detectors relies on thermal diffusion. On one side of the CdZnTe substrate, indium is thermally diff-used to form an N.sup.+ layer, and on the other side a thin Au layer is deposited to provide a P.sup.+ contact. The drawback of this approach is that indium, in addition to being a donor, forms defect complexes which could degrade the energy resolution. Also, detectors fabricated by this process exhibit poor stability.
Accordingly, it is an object of the invention to provide a semiconductor P-I-N detector that has low leakage current and that can operate at room temperature. These and other objects will be apparent from the description that follows.